1. Field of the Invention
The present invention relates to a method and an apparatus for electro-kinetically decontaminating the soil contained in a radioactive waste drum.
2. Description of the Prior Art
Patent applications for the technique similar to the present invention have been filed since 1989, and the last patent was issued in 1998 under U.S. Pat. No. 5,725,752.
Between 1981 and 1985 in the United States, there were developed a method for extracting water from a sludge by using a dynamic electric current, a method for drying wet soil, and a method for adjusting the water flow amount or for reversing the water flow direction. Patents were issued for these techniques (U.S. Pat. Nos. 4,305,800, 4,382,341, and 4,479,857). Between 1976 and 1993 in Japan, a method for extracting water from soil was developed, and patents were issued (02136435 JP A1). Besides the environmental industries, the Hewlett-Packard company utilized the electroosmosis to develop an ink distribution unit for the ink jet printer. Patent applications for this technique were filed in the U. S. and Japan.
To describe the patents related to the present invention, R. F. Probstein et al. filed a patent application in 1989, and obtained a patent in 1991 (U.S. Pat. No. 5,074,986). The technique was on decontaminating the soil site, and the title of the invention was "Electroosmosis Techniques for Removing Materials from Soil". The main claim was on electrically decontaminating the contaminated soil site, and the related claims were on the voltage gradients, on the electrodes, and on the purging liquid supply method.
Almost simultaneously with R. F. Probstein et al., Y. B. Acar et al. obtained a patent (U.S. Pat. No. 5,137,608) under the title of "Electrochemical Decontamination of Soils or Slurries". The total number of claims were 33, and the technique was on an inert anode (containing graphite) and a non-reactive cathode (containing graphite). The object to be decontaminated is metals (such as Pb ions), organic materials, and radioactive nuclides. The Pb ions are recovered at the cathode. The main electrochemical process within the soil includes the steps of: (1) transmitting direct electric current through the soil between an inert anode and non-reactive cathode; (2) supplying a first liquid which comprises water to the soil near said anode; and (3) introducing a nutrient into the soil thorough electroosmosis.
J. P. Bibler obtained in 1993 a patent under the title of "Method and Apparatus for Removing Ions from Soil" (U.S. Pat. 5,190,628). The technique is on selectively removing ions from the soil. That is, a permeable membranes impregnated with an ion exchange resin that is specific to one or more species of chemical ions are inserted into ground. Then electrodes are installed near the membrane, and a voltage is supplied to them. Preferably, the resin exchanges ions of sodium or hydrogen for ions of metal that it captures from soil area. When the permeable membrane is saturated with the metal ions, the potential applied across electrodes is discontinued and membranes are preferably removed from soil for storage or recovery of the ions. The claims state the following matters. That is, there is provided a method for removing one or more species of ions from an area of soil groundwater. That is, (1) placing at least two electrodes in spaced relation with respect to each other and to said area of soil; (2) placing a permeable barrier adjacent each of said at least two electrodes and said area of soil, said barrier adapted to pass said groundwater, said barrier carrying an ion exchange resin, said resin being selective for said at least one species of ions; and (3) applying a potential across said electrodes, said potential causing said at least one species of ions to migrate with respect to said electrodes, thereby allowing said ion exchange resin carried in said permeable barrier to capture said species. The main species removed is assumed to be a mercury ion, and at least two titanium electrodes are installed. The first and second membranes are spaced approximately 10 feet apart and said electric potential is approximately 60 volts DC.
H. R. Chang et al. obtained a patent in 1993 under the title of "Surfactant Augmented In-situ Removal of Poly-chloro-benzenes from Soil by Electroosmosis" (U.S. Pat. No. 5,240,570). In this method, the contaminants such as poly-chloro-benzenes are electro-osmotically extracted from soil at the contamination site. By adding a low concentration surfactant solution, the decontamination capability is reinforced.
R. A. Jacobs obtained a patent in 1995 under the title of "Removal of Contaminant Material from a Soil Site" (U.S. Pat. No. 5,415,744). In this method, at least one anode is positioned in the soil site, and a plurality of cathodes are disposed around the anode. Then a voltage gradient is established between the anode and the cathodes. Owing to the electroosmosis, the solution at the anode moves to the cathodes. That is, the contaminant materials are moved together with the solution, and are removed at the cathodes. The total number of the claims was 19, and the method includes the steps of: (1) positioning one anode at a first location within said soil site; (2) positioning a plurality of only cathodes at second locations around said anode; (3) establishing voltage gradients between said anode and the said plurality of cathodes; (4) supplying a non-contaminated solution to the anode so as to make the solution and the contamination substances move toward the cathodes; and (5) removing said material from said cathodes. Here, the number of the cathodes should be at least three, and the cathodes should be disposed symmetrically relative to the anode. The cathodes are disposed in a circular form. At the step 5, the solution together with the contamination substances can be removed through separate paths for the respective cathodes, or can be combinedly removed.
W. Pool of Netherlands obtained a patent in 1995 under the title of "Process for the Electroreclamation of Soil Material" (U.S. Pat. No. 5,433,829). The content of this invention is similar to the other ones, but is characterized in that: (1) one or more anodes and one or more cathodes are used, and different levels of voltages are applied between different electrodes; and (2) the contaminants are removed both at the anode and at the cathode unlike the other inventions. The total number of claims was 11. Claim 1 defines as follows. That is, an anode and a cathode are installed within a soil which is contaminated with heavy metals and cyanides. Then an electric current is made to flow between the electrodes so that the contaminants would be gathered to the electrodes. Specifically, claim 1 includes the steps of: (1) inserting an electrode into a porous container to make the electrode serve as an anode (or a cathode) so as to make an electric current flow between this electrodes and another electrode (cathode or anode); (2) installing the electrode into the soil; (3) injecting an electrolytic solution into the electrodes through a cathode circulation system and through an anode circulation system; (4) supplying a dc current to the electrodes to make the contamination substances move to the electrodes (electro-migration); and (5) removing the contamination substances from the anode and the cathode.
Further, continuous adjustments are carried out on the pH level of the circulation system, the electrical conductivity, temperature and current density within the solution. Other claims contain the following contents. That is, an acid is added to adjust the acidity of the solution so as to improve the decontamination effect. The anode is made of a noble metal, and the cathode is made of graphite. The heavy metals to be removed are ions of Cu, Pb, Zn, Ni, Co, Fe, Sr, Mg and Ca. In this method, the migration of the contaminants can also be prevented by the application of electrical method.
R. J. Griffith et al. obtained a patent in 1996 under the title of "Electric Field Method And Apparatus for decontaminating Soil" (U.S. Pat. No. 5,584,980). The subject matter of this invention is as follows. That is, an in-situ decontamination is carried out by using a plurality of adjacent electrodes. This invention offers a low cost decontamination. The total number of claims is 33, and claim 1 includes the steps of: (1) arranging a plurality of small adjacent electrodes in one rows into the soil to form a planar anode; (2) arranging a plurality of small adjacent electrodes in one row into the soil to form a planar cathode; (3) making the distance between the electrodes within one row become 50% as large as the distance between the rows; (4) supplying voltages to the anodes; (5) directing a first fluid containing an additive to the anode assemblies to make the solution move to the cathode based on the electroosmosis; (6) making the remaining solution flow faster than the electroosmotic flow so as to recover it.
In claim 2, a second solution having an additive is injected to the cathode, and the solution coming from the anode is recovered. Besides, the claims contain the following contents. That is, the waste solution is treated to reuse it. The method is applied to the region of soil. The anodes are arranged alternately with the cathodes. A permeable geomembrane is used. Two cylindrical electrodes are coaxially used, so that a solution channel can be formed.
J. G. Sunderland et al. of England obtained patents in 1998 under the title of "Electrokinetic Decontamination of Land" (U.S. Pat. No. 5,725,752, and EP 0 724 492 B1). The subject matter of this invention is as follows. That is, the soil is decontaminated by applying a DC electric field across electrodes. At least one anode and at least one cathode are installed in or on said soil, and then, a dc electric current is supplied. Here, a typical anode assembly for use in the method of the invention comprises the carbon felt anode material wrapped around a suitable current feeder and one or more electrolyte feed means. And, the cathode assembly for use in accordance with the method of the invention comprises a cathode, which may be, for example of stainless steel or alternatively a carbon felt material like the anode, within a porous housing, the housing allowing the passage of electrolyte therethrough. This is an in-situ decontamination method using an electrode group, and its cost is very low. The total number of claims is 16, and claim 1 is characterized as follows. That is, at least one anode is installed within the soil, and a DC current is supplied to it, so that oxygen would be produced. The anode is wrapped with a carbon fiber, and the carbon fiber directly contacts with the soil. In claim 2, a cathode is disposed within a porous housing through which the electrolyte can pass. The porous cathode housing is made of a ceramic material, a microporous polymer material or a dense cloth. -The anode is made of carbon, active carbon, or carbon fiber. When a DC current is supplied to it, a colloidal carbon is formed, and moves together with the water owing to the electric field. An electrolyte is supplied through tiny channels which are formed within the carbon anode layer. The cathode is made of stainless steel, nickel, carbon felt and titanium suboxide. The additive for improving the decontamination efficiency is composed of water soluble electrolytes, non-ionic surfactants, cationic surfactants, complexing agents, polyelectrolyte leachants, inorganic acids and organic acids. Between the anode and the cathode, there is at least one bore hole. And, at least one bore hole is positioned outside of an area between the cathode and the electrode.
Yamazaki Teruhiko and Katsura Tsutomu filed patent applications respectively in 1991 and 1993 (19910831 JP 03-244356, and 19930206 JP 05-42197). The titles were "Method for Disposing Polluted Soil, And Its Equipment" and "Method And Device for Treating Water-containing Soil On Site by Electroosmosis".
Orito Shigeru filed a patent application in 1995 (19950411 JP 07-110103), and the title of the invention was "Electrochemical Treatment". In this invention, without using a complicated apparatus, a contaminated soil or sludge is separated and treated. That is, the contaminated soil or sludge is put into between an anode and a cathode, and a solution is let to flow on it, while supplying electric current. Then the harmful substances move between the anode and the cathode. These substances are extracted and electrolyzed.
Takagi Kazuhiro at al. filed a patent application in 1995 (19951227 JP 07-341207), and the title of the invention was "Method for Removing Salt from Saline Soil". The subject matter of the invention is as follows. That is, when electroosmotically decontaminating the soil, metal components are deposited on the electrode, and this problem is solved. That is, the deposition of the metals is inhibited by adding a complexing agent into the solution.
Further, it was found that Monsanto Company (St. Louis) filed a patent application (Application No. 95-70601, and Laid-open No. 96-703390) in Korea. This is same as U.S. Pat. No. 5,476,992 (In-Situ Remediation of Contaminated Heterogeneous Soil). It is found that the total number of claims is 3 in a Korean patent, and claim 1 defines that a chemical agent is added to clean a contaminated soil. Claim 2 defines that an electrical method is employed to clean a contaminated soil. Claim 3 defines that a water pressure flow is induced based on the water pressure gradient to clean a contaminated soil. Particularly, in claim 2, a dc current is supplied between a first electrode and a second electrode for the following purposes. (1) An electroosmotic flow is induced from the second electrode to the first electrode, or (2) ionic migration occurs toward the electrode having the opposite charges, or (3) ionic contamination substances are made to migrate from the second electrode to the first electrode by electroosmosis and toward the electrode having the opposite charges by electromigration. Here, (a) The first electrode is disposed at a first edge of the contaminated soil area, and the second electrode is disposed at the opposite edge of the contaminated soil area. (b) Th e first electrode is disposed at an edge of the low transmissive soil area.